The era of modern bridge building really starts in 1779 with the construction of the Ironbridge at Coalbrookdale in Shropshire (Figure 1).

Figure 1: The Ironbridge at Coalbrookdale

What makes this bridge so important is the material used and the way it was designed. It was made of cast iron from a blast furnace, and designed in large pieces which could be fitted together and hoisted into position relatively easily. The joints in the structure are similar to those used in woodwork, using wedges, chocks and dovetails to give firm connections (Figure 2).

Figure 2: Wedges, chocks and dovetails give strong connections

Some parts of the bridge cracked shortly after construction since cast iron is very brittle when bent, and the ground upon which it was built shifted. Repairs fixed the problem, and the bridge survives to this day (with some reinforcement). The concept is successful because the material is in compression, exploiting the greatest strength of the iron. The same principle had been used since at least Roman times in stone bridges. The design was imitated elsewhere with some success, both in Britain and abroad, and cast iron was also used widely in the early Victorian period in buildings. It culminated in the world famous Crystal Palace, where cast iron columns support wrought iron beams which in turn support wooden floors. However, it is remembered for the glazing which forms the exterior roof cladding (Figure 3).

Figure 3: Building the Crystal Palace using system methods

What cannot be so easily appreciated is the way it was built in the remarkably short time of 3 months, to meet the deadline for the opening of the 1851 Exhibition. The building was the largest in the world, and was disassembled shortly after the Exhibition ended and reconstructed in Sydenham. It is an example of system building, using pre-fabricated standard parts which were fitted together on site using methods more akin to a factory than a building site. It epitomises the Industrial Revolution both as a building made using methods of mass manufacture, and for the multifarious contents shown at the Exhibition.

Copyrighted imageCredit: Production team

However, materials could be misapplied in bridge structures, and disaster could follow very quickly. During the expansion of the railways in the 1830s and 40s, many bridges were needed to cross rivers and estuaries to create a viable network. Growth of the rail network was vital for industrial growth by opening up local markets to a wider audience, and for transport of energy resources, especially coal, and materials such as natural fibres and iron itself.

One such bridge across the Dee at Chester, was made using cast iron beams. It cracked in the middle while a train was passing over, and the carriages fell into the tidal river below killing seven (Figure 4). Robert Stephenson claimed that the train hit the bridge and caused the collapse, but the local inquest jury found the design to be faulty, and condemned it in very forthright terms. The tragedy happened just four years before the Great Exhibition, and led to changes in the way cast iron was used, and greater use of wrought iron, which is much tougher and can be used in beams (such as those in the Crystal Palace).

Figure 4: The collapse of the Dee Bridge, Chester 1847

The rail network helped industry transport its products, and grew yet further in the century, further stimulating industrial growth (a process known as positive feedback). Workers were more mobile as well, so that special skills could be traded over a much bigger area than before. However, the larger estuaries provoked problems, especially by their size or the height of the surrounding country. The Forth and Tay estuaries in particular, proved to be substantial obstacles to the network, especially if lines from Edinburgh to the North were to be shortened. The story of the bridging of the two estuaries is both tragic and heroic. The first attempt was made by bridging the Tay with a 2 mile long bridge, then the longest in the world. It survived for just over two years before falling in a severe gale on December 28th, 1879, taking with it a whole train. There were no survivors from the 75 passengers and crew (Figure 5). During its short life however, it stimulated the growth of the textile industry in Dundee by providing coal for the steam engines which powered the looms and other textile machinery. So why did it collapse?

Figure 5: The fallen Tay Bridge, as shown in the popular press

Copyrighted imageCredit: Used with permission

Some have suggested that the "High Girders" section (about half a mile in length) was blown over by the storm, but the original inquiry in 1880 found that the bridge had been badly designed, constructed and maintained. The tall piers which supported the main girders was a space frame made from cast iron columns braced together with wrought iron tie bars (figure 6).

Figure 6: Section of the Old Tay bridge showing the High and Low girders.

Although the columns were mainly in compression, the bracing bars were tensioned to lugs cast integral with the columns. The cast iron here was in tension, and much weaker than had been realised at the time it was designed. The lugs broke very easily (Figure 7), and it is likely that the whole structure was oscillating laterally when trains passed over. High winds would have added extra loads onto a structure which had been vibrating for many months before the final crash. Defects showing the weakness of the structure were found during maintenance, but their significance not appreciated. The tie bars had loosened, and bolts came apart, falling onto the pier foundations.

Figure 7: Close-up of fractured lug, with intact lug and wedged joint at right

The demands of local industry assured that the bridge would be replaced, and the new bridge was finished in 1887. It was much more stable laterally since the girders were put on arches, and they carried a double rather than single track (Figure 8). It remains in use safely to this day, together with a new road bridge downstream.

Figure 8: The New Tay Bridge with the piers of the older strcuture still visible to the right

The Forth estuary presented quite different problems. Although the estuary was much narrower, the land either side was much higher, so a tall but shorter bridge was needed. A suspension bridge had been suggested earlier by the same engineer (Bouch) who built the first Tay bridge. When he was blamed for the disaster, the contract was removed and eventually given to Baker and Foster. They proposed a quite different way of crossing the Forth: a cantilever design with three massive piers to support the double rail track (top right).

The entire bridge was built from mild steel, a relatively new material for engineering, but much tougher than cast iron (especially in tension). The structure was held together with rivets, preventing any loosening with vibrations from passing trains. Although the structure is very safe, 57 workers were killed during construction, a sad reminder of the risks taken by Victorian engineers. Steel is also susceptible to rusting in the salty conditions of the estuary, so must be repainted regularly (a fact remembered by popular simile!). Together with the new Tay bridge, journey times decreased substantially, and freight for expanding industry was available at lower cost. The completion of the bridge in 1890 may have signalled the end of the great railway era, but enabled industry in Scotland to thrive and prosper.

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